Dark-Activated Alarm With Pulsed Tone Output Circuit

NOR gates a and b form a low-frequency oscillator that is activated when the CDS cell, under dark conditions, causes NOR gate a to see a logic zero at one input. This low-frequency (10 Hz) gates a high-frequency oscillator (c and d) to oscillate at around 1000 Hz. Rl can be varied to change the pulse rate and R2 to change the tone.